A vinyl chloride resin has been widely employed in various fields, since it can produce a processed article having excellent physical and chemical property. But there are various problems in processing such as a narrow processing temperature and slow melting, since a processing temperature is close to the decomposition temperature thereof.
Many technologies, which overcome the above-mentioned processing problems, have recently been known. As a typical technology, examples thereof are a method of adding a plasticizer to a vinyl chloride resin, a method of employing a vinyl chloride resin copolymerized with another monomer such as vinyl acetate, and a method of mixing a vinyl chloride resin with another resin component.
However, all these methods can not satisfactorily improve processability with keeping excellent physical and chemical property which the vinyl chloride resin originally has. For example, in case of adding a plasticizer to a vinyl chloride resin, or employing a vinyl chloride resin copolymerized with another monomer, physical property of the processed article changes drastically. Most of the methods of mixing a vinyl chloride resin with another resin component is a method of lowering processing temperature by lowering melt viscosity at mold processing. It seemingly improves fluidity of a vinyl chloride resin, but indeed gelation of a vinyl chloride resin becomes unsatisfactory due to consumption of mixing energy through fluidity. Therefore, in the application in which transparency is required, there arises a problem that physical property thereof is inferior to that of the completely gelled vinyl chloride resin even if the processed article prepared therefrom seemingly becomes transparent.
In order to solve the above-mentioned problems, Japanese Examined Patent Publication No. 5311/1965 discloses a method of adding, as a processing aid, a copolymer of methyl methacrylate as a main component having comparatively high molecular weight. But the added processing aid is apt to be left as non-gelled object (usually called “fish eye”) in the processed article, it is easy to lose appearance of molding. And the other effects of adding a processing aid, such as gloss improvement of product, improvement of secondary processability, and decrease of specific gravity by foam molding, are not satisfactorily revealed.
And Japanese Examined Patent Publication No. 49020/1977 and No. 2898/1978 disclose a composition which contains a processing aid prepared by two step polymerization. At first a major amount of methacrylate ester is polymerized followed by polymerization of a minor amount of acrylate ester in the presence of a latex of poly(methyl methacrylate) or a copolymer of a major amount of methyl methacrylate and a minor amount of acrylate ester. The method was effective in prevention of the above-mentioned non-gelled object, but the effect thereof was not satisfactory. And depending on mixing and application, there arised many problems caused by unsatisfactory dispersion of a processing aid in a vinyl chloride resin composition, such as decrease of molding transparency of a vinyl chloride resin, decrease of gelation ratio, and decrease of secondary processability such as elongation at a high temperature.
Foam molding has recently attracted considerable attention as a method of decreasing a density of a processed article and lowering cost of molding. In the above-mentioned foam molding of a vinyl chloride resin, there has been known a method of employing a processing aid containing methyl (meth)acrylate as a main component and a foaming agent. But it was difficult to raise expansion ratio to not less than 3 times with keeping uniformity of molding appearance, and uniformity and minuteness of foaming cell. On the other hand, formed molding of a vinyl chloride resin having higher expansion ratio has been strongly demanded in the market.
As disclosed in Japanese Examined Patent Publication No. 5311/1965, it has been known that processability can be improved by adding a copolymer comprising methyl methacrylate as a main component having high molecular weight to a vinyl chloride resin. It can be expected that the higher the molecular weight thereof becomes, the higher the effect thereof becomes. But in case of employing the processing aid having merely raised molecular weight, not only transparency, but also a gel ratio thereof may be lowered and secondary processability such as elongation at a high temperature as well. Therefore, it is not always practical.
Influence of a micro structure of the processing aid in the final composition has never been understood in detail. But it should be the reason that dispersibility becomes low with increasing the molecular weight thereof.